In measurement of Raman spectra of examination subjects, such as biological tissue, optical fibers have been used to radiate laser beams and receive Raman scattered light. Because constituent materials, including silica etc., forming such an optical fiber also generate Raman scattered light due to a laser beam being guided by the optical fiber, an observed spectrum acquired by using the optical fiber also contains a Raman spectrum of the optical fiber as noise. Furthermore, the observed spectrum contains, as noise, background light signals originating from light other than Raman scattered light of the examination subject, such as Rayleigh scattered light and autofluorescence. To overcome this problem, methods for reducing noise contained in observed spectra have been proposed (refer to, for example, PTL 1 and 2 and NPL 1).
PTL 1 and 2 each disclose a Raman probe in which an optical filter that cuts Raman scattered light generated by an optical fiber and that transmits only a laser beam is provided at the distal end of a radiating optical fiber and an optical filter that transmits Raman scattered light of biological tissue and that cuts reflection light of the laser beam is provided at the distal end of a light-receiving optical fiber. According to such a configuration, a Raman spectrum of the examination subject can be acquired, the spectrum being free of the Raman spectrum of the optical fiber and exhibiting a high signal-to-noise ratio.
NPL 1 discloses a method in which the spectrum waveform of background light contained in an observed spectrum is fitted with a third- to seventh-order high-order polynomial, and the background light signal contained in the observed spectrum is removed by subtracting the fitted waveform from the observed spectrum.